This invention relates to an orthodontic bracket for use in orthodontic therapy.
Conventionally, an orthodontic bracket is generally made of a metal material such as stainless steel or the like. FIGS. 5 and 6 show a conventional example of such an orthodontic bracket 1. A flat-shaped bracket base 2 is united with ligature wire clamping wings 3 to 6 of hook-shapes (L-shapes) at both ends of an upper surface of the bracket base back to back in an opposite manner and which number four in total. A thin metal wire (an archwire) 8 which clamps the orthodontic bracket 1 on a tooth surface is passed through narrow U-shaped slots 7,7 between the back-to-back opposite wings 3 and 4, 5 and 6. A thin wire (ligature wire) 9 is clamped to the ligature wire clamping wings 3 to 6 to fasten the archwire 8 in the slots 7,7. An upper surface of the bracket base 2 between the ligature wire clamping wings 3 to 6 is perpendicular to the slots 7,7 to form a groove 10 which has a width and a depth wider and somewhat deeper than the slots 7,7. A space between extremities of the ligature wire clamping wings 3 to 6 and the bracket base 2 form ligature wire clamping grooves 11 for clamping the archwire 8. Such a metallic orthodontic bracket 1 is excellent in strength and function but is disadvantageous in that it is conspicuous due to its metallic luster and is unattractive.
Recently, studies have been made regarding applications of various ceramics, polymers, and composites thereof to the bracket for purpose of improving its appearance. FIGS. 7 and 8 show a general configuration of an orthodontic bracket 12 which is made of such materials and which resembles the metallic bracket 1. However, it is to be noted that, a pair of slot-forming walls 13 which define slot 7 project continuously from the bracket base 2 over the entire length of the base in the direction of the ligature wire, and the groove 10 of the metallic bracket is generally omitted.
In use of the above orthodontic brackets 1 and 12, a bottom surface of the bracket base 2 is adhered and fixed to a surface of a tooth. The archwire 8, of a rectangular cross section, is passed through the slot 7 and fixed to the bracket base 2 by the ligature wire 9. With this construction, the teeth are subtly moved along the archwire 8 by applying a load such as torsion, bending, tension, or the like to the archwire 8 and by transmitting the load to the teeth. Accordingly, excessive friction between the inside surface of the slot and the surface of a wire surface reduces the efficiency of movement of the teeth and undesirably lengthens the treatment period.
Furthermore, the orthodontic bracket 12 of a nonmetallic material is weak in toughness and is subject to breakage and cracking during medical treatment in comparison with the metallic bracket of, for example, stainless steel. Various loads are applied to the orthodontic bracket 12 by the archwire 8, and the like. Main loads which cause the breakage to occur are classified into two categories as illustrated in FIGS. 9(a) and (b). In the load category as illustrated in FIG. 9(a), the stress tends to be concentrated at a corner 14 at the bottom of the slot due to reaction force caused by the torsion of the archwire 8. A crack is often produced from the corner 14 towards the bottom of the ligature wire clamping wing 3, namely, in narrow or waist portion 15. On the contrary, in the load category, illustrated in FIG. 9(b), the stress is liable to be concentrated at a bottom portion 16 on the lower surface of each ligature wire clamping wing 3 due to tension caused by the ligature wire 9. As a result, a crack is produced from the bottom portion 16 towards the slot 7. The fracture strength of the ligature wire clamping wing 3 is approximately proportional to the square of the distance between the corner 14 and the bottom portion 16 of the lower surface.
Another problem of the bracket 12 made of ceramic or polymer is that the efficiency of movement of the teeth is low and that a long period of time is required for medical treatment because the friction of the archwire 8 is larger than that with the metal bracket.
In addition, the bottom surface 17 of the slot 7 is substantially flush with the bottom portion of the lower surface in FIG. 9. In FIGS. 9a and 9b numeral 18 is a tooth and 19 is cement.